This message is displayed in the console log: "CASKC1001W Warning: Concurrent request limit exceeded 'n' times" (where 'n' is a number).

This message means that the number of concurrent users has exceeded the currently-configured license limit for this region (i.e. only a restricted number of jobs have been run at any one time but more were requested).  The "concurrent user count" is defined as the number of clients that are currently submitting a request for processing within Enterprise Server (ie. at the same time).

The message is followed by another message showing the limit and how this has been exceeded, for example:
CASKC1001W Warning: Allocated license limit exceeded 0000000006 times
CASKC1002I ES allocated license limit: 0000000004 units, peak usage: 0000000010 units

When a region is started, the currently-configured license limit for this region is displayed in the following entry:
CASKC1000I ES concurrent request limit: 0000000004

This limit is the value in the "Requested Licences" field which is set in the Enterprise Server Admin Page. Go to Server > Properties > General Tab > 'Startup Options'.
To prevent this error message from occuring it is necessary to set the value to the number of users/jobs that are required (or expected) to be run/processed concurrently.
It is a good idea to add 20% of the total to ensure that you do not exceed the limit.  However, the value can only be increased up to the limit imposed by the current license.